1. Field of the Invention
The present invention relates to a structure of a p-electrode at the light-emerging side of a light-emitting diode, and particularly to a structure of the p-electrode improved so as not to obstruct the passing of light in a light-emitting diode using a wide-bandgap semiconductor such as ZnSe or GaN.
2. Description of the Background Art
A light-emitting diode (hereinafter referred to as LED) has an n-electrode on one of the two main surfaces (top and bottom surfaces) and a p-electrode on the other. In the case of a surface-illuminating-type LED, either surface is used as the light-emerging surface. The electrode, usually made of metal, is required not to obstruct the passing of light.
An LED such as an AlGaAs-based LED has a structure in which a p-type GaAs contact layer, having a sufficient electrical conductivity, is placed on the pn junction, and a small-area dot- or ring-shaped p-electrode is placed on the contact layer. In this specification, the narrow p-electrode is also called a bonding electrode because it is used for wire bonding.
Because the p-type GaAs contact layer has low resistance, it has the ability to diffuse a current injected from the small-area p-electrode, making the current density nearly uniform under the contact layer. In other words, the p-type GaAs contact layer desirably functions as a current-diffusing layer. Therefore, when the narrow bonding electrode is formed on the p-type contact layer, this simple structure can sufficiently diffuse the current.
However, when an LED is produced by using a material that makes it difficult to obtain a low-resistance p-type semiconductor, the semiconductor itself cannot sufficiently diffuse the current. With a GaN- or ZnSe-based wide-bandgap semiconductor, it is difficult to produce a p-type semiconductor, particularly a low-resistance p-type semiconductor. This is the reason why an LED using such a semiconductor has difficulty in obtaining a low-resistance p-type contact layer. As a result, the LED cannot be provided with a current-diffusing layer.
With an LED having no current-diffusing layer, the bonding electrode for wire bonding alone cannot diffuse the current sufficiently in the p-type contact layer. Consequently, the current is localized at the portion underneath the electrode. As a result, the effective area of the light-emitting section is decreased, notably decreasing the luminous efficiency. In addition, the current crowding decreases the life of the LED. To prevent the current concentration at the portion directly under or in the vicinity of the bonding electrode, an LED having no current-diffusing layer usually has a structure in which an extremely thin semi-transparent metal electrode made of Au, for example, is formed on the entire surface of the light-emitting region. In this structure, the bonding electrode is placed on a portion of the light-emitting region. Au can provide ohmic contact with the p-type contact layer.
In this specification, the thin metal electrode is referred to as “a semi-transparent thin-film metal electrode” to distinguish it from the bonding electrode. In other words, the p-electrode has a dual structure composed of the bonding electrode and the semi-transparent thin-film metal electrode. Whereas the bonding electrode does not transmit light, the semi-transparent thin-film metal electrode can transmit light to a certain extent. Consequently, light generated at the pn junction can pass through the semi-transparent thin-film metal electrode to exit to the outside.
However, although the semi-transparent thin-film metal electrode covering the entire p-type contact layer can transmit light, its transmittance is low. As a result, in a GaN- or ZnSe-LED, part of the upward light emitted from the active layer is absorbed or reflected by the semi-transparent thin-film metal electrode. The axial light intensity decreases by the amount of the absorption and reflection losses.
The absorption and reflection at the semi-transparent thin-film metal electrode decreases the amount of light emerging from the LED to the outside, decreasing the brightness accordingly. To increase the light intensity (brightness), it is necessary to decrease the absorption and reflection at the semi-transparent thin-film metal electrode. In other words, it is necessary to increase the transmittance of the metal electrode. To achieve this purpose, the thickness of the metal electrode can be decreased. However, if the metal electrode is excessively thin, the electrical resistance in the lateral direction increases, decreasing the effect of current diffusion by the metal electrode. For example, when Au is used as the metal electrode, if the thickness is decreased to 5 nm, a sufficient current cannot flow into the Au electrode due to the increased resistance. It is desirable that the Au electrode has a thickness of at least 10 nm, preferably about 20 nm. Although thin, the Au electrode having a thickness of about 20 nm has a transmittance as low as 40% to 60% or so. In other words, about half the amount of the light is decreased by the presence of the Au electrode.
The published Japanese patent application Tokukai 2001-148511, entitled “Semiconductor light-emitting diode,” has disclosed a mesh-shaped electrode structure of an LED in which (a) a mesh-shaped metal electrode is formed on the n-type contact layer, (b) a transparent, electrically conductive oxide electrode layer made of indium tin oxide (hereinafter referred to as ITO) is formed on the mesh-shaped metal electrode, and (c) a bonding electrode is formed at the center of the oxide electrode layer (the bonding electrode is referred to as a pad electrode in Tokukai 2001-148511). In other words, the n-side electrode structure is composed of the bonding electrode, the ITO layer, the mesh-shaped metal electrode, and the n-type contact layer.
ITO provides ohmic contact with the n-type contact layer. An injected current sufficiently diff-uses through the thick ITO layer and proceeds into the n-type contact layer with a uniform distribution. Because the ITO layer has an enough thickness, it has the ability to diffuse the current. Therefore, the semiconductor layer (n-type contact layer) is not required to diffuse the current.
Although electrically conductive, ITO is less conductive than Au. Consequently, the transparent electrode is as thick as 600 nm to 1 μm. Because the ITO is less conductive, the bonding electrode must be placed at the center of the ITO layer in order to obtain the uniform current distribution throughout the chip. If the bonding electrode is placed at a portion close to the periphery of the ITO layer, the current will not flow sufficiently at the opposite portion.
The GaN-LED provided with the ITO transparent electrode without using a mesh-shaped metal electrode has a problem in that the junction barrier between the ITO electrode and the n-type contact layer (n-GaN) is so high that the forward voltage drop becomes as high as seven volts. As a result, a low-voltage drive cannot be performed.
In the structure disclosed in the foregoing Tokukai 2001-148511, the mesh-shaped metal electrode (Au/Ge) is placed on the peripheral portion of the n-type contact layer excluding the central portion for placing the bonding electrode. With the above-described GaN-LED having no mesh-shaped metal electrode, because the current flows directly from the ITO layer to the n-GaN contact layer, the voltage drop becomes as high as seven volts. On the other hand, in the case of the above-described structure disclosed by Tokukai 2001-148511, the current first flows from the ITO layer to the mesh-shaped metal electrode producing no voltage drop, and then flows from the mesh-shaped metal electrode to the semiconductor layer (n-GaN contact layer) producing a voltage drop of no more than about three volts.
As explained above, in the structure disclosed in the Tokukai 2001-148511, the current flows in order of the bonding electrode, the ITO layer, the mesh-shaped metal electrode, and the n-type contact layer. The n-electrode is placed at the side from which the light emerges. As opposed to this, as in the present invention, there is a structure in which the p-electrode is placed at the side from which the light emerges. The n-type contact layer sufficiently absorbs dopants, increasing the carrier density. This high carrier density sufficiently reduces the resistance, facilitating the current diffusion. However, this process cannot be applied to the p-type contact layer aimed at by the present invention, for example.
Consequently, researchers and engineers are required to develop a surface-illuminating-type LED causing the light to exit through the p-electrode, particularly an LED using a wide-bandgap semiconductor such as ZnSe or GaN, in such a way that the LED has an improved light transmittance at the p-electrode in contact with the p-type contact layer to increase the optical output.